There have been proposed refrigeration cycle apparatuses in which an expander recovers the expansion energy of a working fluid, and the recovered energy is used for a part of the work of the compressor. As one of such refrigeration cycle apparatuses, a refrigeration cycle apparatus using an expander-integrated compressor is known (see Patent Literature 1).
FIG. 28 shows a conventional refrigeration cycle apparatus using an expander-integrated compressor. This refrigeration cycle apparatus includes a compressor (compression mechanism) 201, a radiator 202, an expander (expansion mechanism) 203, and an evaporator 204. These components are connected to each other by pipes so as to form a main circuit 208. The compressor 201 and the expander 203 are coupled together by a shaft 207. A motor 206 for rotationally driving the shaft 207 is disposed between the compressor 201 and the expander 203. The compressor 201, the expander 203, the shaft 207, and the motor 206 constitute the expander-integrated compressor.
This refrigeration cycle apparatus further includes a secondary circuit 209 that is connected to the main circuit 208 so as to be provided in parallel to the expander 203. The secondary circuit 209 branches from the main circuit 208 between the outlet of the radiator 202 and the inlet of the expander 203, and merges with the main circuit 208 between the outlet of the expander 203 and the inlet of the evaporator 204. A working fluid flowing through the main circuit 208 expands in the positive-displacement expander 203. The working fluid flowing through the secondary circuit 209 expands in an expansion valve 205.
The working fluid is compressed by the compressor 201. The compressed working fluid is delivered to the radiator 2, and cooled in the radiator 202. The working fluid expands in the expander 203 or the expansion valve 205, and then is heated in the evaporator 204. The expander 203 recovers the expansion energy of the working fluid, and converts the recovered energy into the rotational energy of the shaft 207. This rotational energy is used as part of the work for driving the compressor 201. As a result, the power consumption of the motor 206 is reduced.
How the refrigeration cycle apparatus operates when the expansion valve 205 is fully closed will be described.
First, the suction volume of the compressor 201, the suction volume of the expander 203, the rotational speed of the shaft 207 are denoted as Vcs, Ves, and N, respectively. In this case, the volumetric flow rate of the working fluid at the inlet of the compressor 201 is expressed as (Vcs×N). The volumetric flow rate of the working fluid at the inlet of the expander 203 is expressed as (Ves×N). Since the mass flow rate of the working fluid in the secondary circuit 209 is zero, the mass flow rate thereof in the compressor 201 and that in the expander 203 are equal to each other. If this mass flow rate is denoted as G, the density of the working fluid at the inlet of the compressor 201 is expressed as {G/(Vcs×N)}. The density of the working fluid at the inlet of the expander 203 is expressed as {G/(Ves×N)}. Based on these formulas, the ratio between the density of the working fluid at the inlet of the compressor 201 and that at the inlet of the expander 203 is expressed as {G/(Vcs×N)}/{G/(Ves×N)}. That is, the density ratio (Ves/Vcs) is always constant regardless of the rotational speed of the shaft 207 (constraint of constant density ratio).
FIG. 29 shows a Mollier diagram of a CO2 refrigeration cycle. The compression process in the compressor 201, the heat radiation process in the radiator 202, the expansion process in the expander 203, and the evaporation process in the evaporator 204 correspond to AB, BC, CD, and DA, respectively. The ratio between the density of the working fluid at the inlet of the compressor 201 (Point A) and that at the inlet of the expander 203 (Point C) is (Ves/Vcs). If the density at Point A is ρ0, the density ρc at Point C is (Vcs/Ves)ρ0. When the density ρ0 of the working fluid at the inlet of the compressor 201 (Point A) is constant, the state of the working fluid at the inlet of the expander 203 (Point C) always changes along the line that satisfies the relationship of ρc=(Vcs/Ves)ρ0. That is, the temperature and pressure of the working fluid at Point C cannot be controlled freely. The refrigeration cycle has an optimum high pressure at which the highest coefficient of performance (COP) is achieved at a certain heat source temperature (for example, an outside air temperature). Therefore, if the temperature and pressure cannot be controlled freely, it is difficult to operate the refrigeration cycle apparatus efficiently.
There have been several proposals to avoid the constraint of constant density ratio. For example, in the refrigeration cycle apparatus shown in FIG. 28, the constraint of constant density ratio can be avoided by opening the expansion valve 205 to allow a part of the working fluid to flow into the secondary circuit 209. This method, however, has a problem in that the expansion energy of the working fluid flowing through the secondary circuit 209 cannot be recovered, which reduces the effect of improving the COP.
Patent Literature 2 discloses an expander including an auxiliary chamber that can communicate with an expansion chamber. With this expander, the volumetric capacity of the expansion chamber can be increased or decreased by increasing or decreasing the volumetric capacity of the auxiliary chamber. The suction volume of the expander Ves changes with an increase or a decrease in the volumetric capacity of the expansion chamber. Thus, the constraint of constant density ratio can be avoided. Nevertheless, this expander has a problem in that the working fluid remains in the auxiliary chamber. It also has another problem of sealing a piston for increasing or decreasing the volumetric capacity of the auxiliary chamber.